Tomographic atom probe comprising an electro-optical generator of high-voltage electrical pulses

ABSTRACT

A tomographic atom probe uses electrical pulses applied to an electrode in order to carry out evaporation of the sample being analyzed. In order to produce these electrical pulses, the tomographic atom probe comprises a high-voltage generator connected to an electrode by an electrical connection comprising a chip of semiconductor material. The probe also comprises a light source which can be controlled in order to generate light pulses which are applied to the semiconductor chip. Throughout the illumination, the chip is rendered conductive, which puts the high-voltage generator and the electrode in electrical contact so that a potential step is applied to the latter. The probe also comprises means for applying a voltage step of opposite amplitude to the previous step at the end of a time interval Δt 0 , so that the electrode finally receives a voltage pulse of duration Δt 0 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International patent applicationPCT/EP2009/063346, filed on Oct. 13, 2009, which claims priority toforeign French patent application No. FR 08 06550, filed on Nov. 21,2008, the disclosures of which are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The invention relates to the general field of high-voltage pulsegenerators. It more particularly relates to the pulse generators used tocause evaporation of atoms from a sample of material placed in atomographic atom probe with a view to its analysis.

BACKGROUND OF THE INVENTION

An essential characteristic of a tomographic atom probe is the value ofthe mass resolution which can be obtained with this probe. The massresolution of an atom probe is an essential quality which characterizesin particular:

the capability of the probe to unambiguously separate the various masspeaks of the different isotopes of the elements constituting thematerial to be analyzed. This ability to discriminate is commensuratelymore valuable when it relates to neighboring mass peaks with verydifferent amplitudes.

the capability of the probe to increase the measurement accuracy of thecomposition of the material being analyzed, for example an alloy, byreducing the incorporation of detection noise at a mass peakcorresponding to an element of the material being analyzed.

The mass resolution is expressed in the form of a ratio R=m/dm, where mrepresents the mass corresponding to the peak (i.e. to the element) inquestion, and where dm represents the width of the mass peak for anamplitude relating to this given peak. If m is equal to 28 and dm isequal to 0.28, for example, for h=0.5 (width at half height), then R=100at half height.

Consequently, the problem posed for manufacturers of such probes, whichconstitute time-of-flight mass spectrometers, is to obtain the bestpossible spectral resolution. Obtaining a good mass resolution involvescontrolling the motion speed of the material particles from the samplefrom which they are detached, generally in ionic form, to the detector.In other words, the problem posed consists, as is known, in finding away of making it possible for the atoms evaporated in the form of ionsto be detached from the sample with the same initial potential energyand for them then to travel substantially at the same speed to thedetector.

As is also known, the input of potential energy necessary for detachinga few atoms of an atomic layer of the sample is carried out by applyinga voltage pulse of high value (high-voltage pulse), the duration ofwhich is in practice of the order of one nanosecond, in the sampleextraction zone (the tip).

This voltage and the curvature of the end of the sample are sufficientto create an electric field whose strength is enough to obtain the“field-effect evaporation” phenomenon, an effect which is known to theperson skilled in the art. In practice, however, the production of apulse quasi-instantaneously reaching the theoretical potential levelrequired and constantly maintaining this level for a given time, beforelikewise ending almost instantaneously, constitutes a genuine realproblem in the current state of the art. Consequently, pulses whoseupper part has an approximately parabolic shape are generally produced,which leads to a degradation of the mass resolution when such pulses areused in a tomographic atom probe. Varying the amplitude of the pulseapplied to the sample during the evaporation and then during the firstfew nanometers of the trajectory of the emitted ions, thus leads to theappearance of an energy (velocity) spectrum which is fairly broadinstead of a single energy line. The spectral resolution of the atomprobe is therefore contingent on the capacity to produce a square-wavepulse with a high amplitude and steep edges.

FIGS. 1 and 2 illustrate this dependency by simulated mass histogramsfor a tomographic atom probe or “Watap” according to the acronym for“wide angle tomographic atom probe”, the simulated probe having a givenflight length of 0.11 m.

FIG. 1 illustrates a typical favorable case, taken by way of example, inwhich the evaporation pulses are square-wave pulses having a plateau of200 ps and edges of 50 ps.

FIG. 2 in turn illustrates the less favorable case, in which theevaporation pulses are square-wave pulses having a plateau of 200 ps andedges of 1000 ps.

Both the presence of a peak 11, 21 having a certain width and thepresence of a tail 12, 22, which is more extended but of lower level,can be seen in each of the figures. The peak here corresponds to theevaporation produced when the evaporation pulse has reached its maximumlevel (plateau of the pulse), while the tail in turn corresponds to theevaporation produced during the time intervals corresponding to theleading and trailing edges, and for which a loss of energy of theevaporated ions is observed.

Consequently, knowing that the relative amplitudes and durations of thepeak on the one hand, and the tail (part of the pulse lying after thepeak) on the other hand, characterize the resolution of the probe inquestion, and that the higher the amplitude of the peak and the smallerits width are, the greater the resolution of the probe is, it can beseen from FIG. 2 that extension of the rise and fall times of the pulseleads to a degradation of the resolution. In other words, the closer theevaporation pulse approximates a square-wave shape the higher theresolution of the probe can be.

In order to limit this problem, manufacturers are looking for ways ofproducing a constant amplitude plateau with steep edges, that is to sayvery short rise and fall times (typically less than 100 ps) for veryshort pulses (typically less than 500 ps). The known prior art proposesvarious approaches for producing such high-voltage pulses.

For instance, there are high-voltage pulse generation devices using arelay wetted with mercury. The repetition frequency of the pulsesproduced by such devices, of relatively old design, is however extremelylow (of the order of one hundred hertz) in view of the desiredcharacteristics, which makes the analysis of the sample relatively slow.

There are also semiconductor devices, of more recent design, which makeit possible to produce short-duration pulses whose amplitude can be setin a wide voltage range, typically between a voltage of close to 0 V anda voltage of 4 kV. These devices moreover make it possible to obtainrepetition frequencies extending up to a few tens of kilohertz.

However, these performances are obtained at the cost of a degradation ofthe rise and fall times, that is to say the steepness of the edges ofthe pulses produced in this way. In the current prior art, such devicestherefore do not make it possible to produce high-voltage square-wavepulses of short duration, that is to say pulses having steep edges(typically less than 100 ps) and a short (typically less than 500 ps)maximum of constant level.

There are furthermore pulse generator systems capable of producingpulses having the desired temporal characteristics, but a loweramplitude which is fixed or difficult to adjust. These systems aretherefore unsuitable or not very suitable for use in the scope of atomographic atom probe which, by its nature, is intended for theanalysis of different materials, each material requiring the productionof pulses whose voltage value is proportional to the high voltage whichpolarizes the sample.

No known device of the prior art, therefore, is generally satisfactoryin the context of producing atom probes with high mass resolution.

SUMMARY OF THE INVENTION

It is an object of the invention to resolve the specific problem ofproducing high-voltage pulses having the characteristics described aboveand thus to make it possible to improve the mass resolution oftomographic atom probes.

To this end, the invention relates to a tomographic atom probe havingmeans for applying an electrical evaporation pulse of amplitude V_(p)and duration Δt₀ to a sample which is placed at a potential V₀.According to the invention, these means comprise:

an electrode which is placed at an initial potential V_(i) and isconfigured and arranged to apply the electrical pulse to the sample;

a voltage generator capable of producing the voltage necessary forproducing a pulse of amplitude V_(p), the voltage generator beingconnected to the electrode by means of an electrical connection whichcan be open or closed;

means for causing closure of the electrical connection in a given time τso as to apply a voltage step V_(p) to the electrode, these meanscomprising a chip of semiconductor material placed on the electricalconnection between the generator and the electrode, in the vicinity ofthe electrode, and a first source emitting light pulses of wavelength λ₁to the semiconductor chip, said chip becoming conductive and closing theelectrical connection when it is illuminated with a light pulse ofwavelength λ₁, the conduction time being a function of the duration ofthe light pulse applied;

means for applying a voltage step of amplitude (−V_(p)) to the electrodeat the end of a time Δt₀ after closure of the electrical connection, soas to place the electrode at the potential V_(i), said means beingconfigured so that the voltage step is applied in a time τ′substantially equal to τ.

According to a particular embodiment of the tomographic atom probeaccording to the invention, the means for applying a voltage step ofamplitude −V_(p) to the electrode at the end of a time Δt₀ after closureof the circuit consist of a transmission line with a characteristicimpedance Z_(c) connecting the generator to the chip, terminateddownstream of the electrode by an impedance equal to Z_(c).

According to this particular embodiment, the length L₀ of thetransmission line is determined by the value of the time interval Δt₀ inquestion, which time interval is equal to the duration of the electricalpulse produced.

According to another particular embodiment of the tomographic atom probeaccording to the invention, the means for applying a voltage step ofamplitude −V_(p) to the electrode at the end of a time Δt₀ after closureof the circuit consist in a second source emitting light pulses ofwavelength λ₂ onto the semiconductor chip, said chip becoming insulatingand opening the electrical connection when it is illuminated with alight pulse of wavelength λ₂.

According to this particular embodiment, the time interval Δt₀ betweenthe emission of a light pulse of wavelength λ₁ and the emission of alight pulse of wavelength λ₂ determines the duration of the electricalpulse produced.

According to a particular embodiment of the tomographic atom probeaccording to the invention, the electrode on which the electrical pulseis produced is positioned facing that end of the sample on which theevaporation is expected.

According to another particular embodiment of the tomographic atom probeaccording to the invention, the electrode on which the electrical pulseis produced consists of the sample itself.

The device according to the invention makes it possible to formelectrical pulses, in particular high-voltage pulses, having extremelyshort rise and fall times typically of the order of a few picoseconds.This method also makes it possible to control and keep constant themaximum amplitude of this pulse through the duration of the voltageplate. It also makes it possible to create electrical evaporation pulseswhose particular shape makes it possible to optimize the mass resolutionof the atom probe, and in particular its wide angle variant.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the invention will be betterunderstood from the following description, which explains the inventionusing particular embodiments taken as nonlimiting examples and referringto the appended figures, in which:

FIGS. 1 and 2 show two mass histograms presented as illustrations of theresolution problem posed by the electrical evaporation pulse productiondevices of existing atom probes;

FIG. 3 shows a schematic diagram illustrating the way in which theelectrical evaporation pulses are generally produced in an atom probeaccording to the known prior art;

FIG. 4 represents a schematic diagram similar to the diagram of FIG. 3,illustrating the general operating principle of the device according tothe invention with reference to a preferred embodiment;

FIG. 5 shows an equivalent circuit diagram of the sample tip/backelectrode assembly in an atom probe;

FIG. 6 shows an equivalent circuit diagram of the semiconductor chipfulfilling the function of a controlled switch in the preferredembodiment of the device according to the invention;

FIGS. 7 to 10 show illustrations relating to a first preferredalternative embodiment of the device according to the invention;

FIG. 11 shows an illustration relating to a second preferred alternativeembodiment of the device according to the invention;

FIG. 12 shows an illustration relating to a third preferred alternativeembodiment of the device according to the invention.

DETAILED DESCRIPTION

As illustrated in the schematic diagram of FIG. 4, the electrical pulsegenerator device according to the invention comprises firstly a voltagegenerator 41 capable of producing the voltage necessary for producing apulse of amplitude V₂ at an electrode 32, which pulse, in view of thepotential at which the sample is placed, makes it possible to causeevaporation thereof. This voltage generator is connected to theelectrode 32 by means of an electrical connection 46 which can be openor closed.

The device according to the invention also comprises means for causingclosure and opening of the connection 46, so as to apply a voltage stepV₂ to the electrode and place the potential of the electrode, initiallyequal to a value V₁, at V₁+V₂. The means for causing closure of theconnection 46 comprise a controlled switch inserted into the electricalcircuit between the generator 41 and the electrode 32, in the vicinityof the electrode, and means for controlling this switch.

A preferred, but not exclusive, embodiment of these means consists inusing a chip of semiconductor material 42 installed on the circuitconnecting the high-voltage generator 41 and the electrode 32 to whichthe electrical pulse is applied.

The means for actuating the switch constituted by the chip ofsemiconductor material 42 are a light source 43, for example a lasersource, delivering short pulses 44, typically of less than 100 ps, andof high intensity. The length λ₁ of the pulses emitted by the lightsource 43 is determined as a function of the semiconductor used to formthe chip 42. The energy contained in each pulse, typically of the orderof 10 microjoules, is moreover sufficient to set the semiconductor chipin conduction.

According to the invention, the assembly is arranged so that the chip 42is illuminated by the light source 43. To this end, the light pulses 44are applied to the semiconductor chip 42 either directly through atransparent window 45 or by means of an optical fiber or any othermeans.

Thus, by applying a light pulse of determined duration to the chip ofsemiconductor material, the conduction of the chip is made to developnaturally. First, the chip changes in a brief time τ from the insulatingstate to the conducting state. It subsequently remains in the conductingstate throughout the light pulse. Then, after the light pulse isextinguished, the chip returns progressively to the insulating state atthe end of a time τ′.

Setting the chip of semiconductor material in conduction leads to theappearance of a voltage step on the electrode, which is then put at thevoltage V₁. Conversely, the return of the chip to the non-conductionstate leads to the appearance of an opposite voltage step on theelectrode, which is then put at the inactive potential V₀. An electricalpulse is thus produced on the electrode, the duration of which dependson the duration of the light pulse applied and the characteristics ofthe semiconductor constituting the chip. Such a pulse can thereforeadvantageously be very short.

It should however be noted that with such a structure, the time τ′ forreturn to the insulating state is very substantially greater than thetime τ, simply because the time taken to create free carriers within thesemiconductor material when it is illuminated is much less than the timeτ′ taken to eliminate these carriers when the illumination ends.Furthermore, this time t′ cannot be modified by any external means.Consequently, although much shorter than those produced by the meansgenerally employed, the duration of the electrical pulse produced cannotbe completely controlled.

This is why, further to the elements presented in FIG. 4, the deviceaccording to the invention incorporates additional means allowing thewidth of the electrical pulse produced on the electrode to be controlledperfectly, despite the fact that the time for return to the insulatingstate of the chip of semiconductor material, which acts as a switchhere, cannot be completely determined.

The action of these means consists in applying a voltage step ofamplitude −V₁ to the electrode at the end of a time Δt₀ after thecircuit is closed, that is to say in the preferred embodiment after thechip is set in conduction, so as to place the potential of the electrodequasi-instantaneously at V₀. In this way, it is possible to obtain anelectrical pulse whose duration can be shorter (of the order of onehundred picoseconds) than that of the pulses produced with conventionalpulse generators, and whose leading and trailing edges are extremelysteep (a few picoseconds) and have comparable or even substantiallyequal durations.

It should be noted that such means, which act directly on the potentialat which the electrode is placed, may advantageously be associated witha wide variety of means for causing closure and opening of theconnection 46. The advantage offered by using such means is thereforenot limited to the particular case of the preferred embodiment describedabove, in which these means consist of a chip of semiconductor materialand a source producing light pulses.

From a physical point of view, in the preferred embodiment of the deviceaccording to the invention, the high-voltage pulses are thereforeproduced by means of one or more chips of semiconductor material 42(silicon, germanium, gallium arsenide or the like) with a direct orindirect gap. Such elements, which are known for their electro-opticalproperties, become conductive for a short time under the effect of abrief and intense light pulse 44, coming for example from a femtosecondpulse laser generator. Consequently, the wavelength λ₁ of the lightpulse 44 is selected in order to generate free charges byphotoconduction in the semiconductor material. In other words, theenergy of the emitted photons is determined so as to be greater than thegap of the material used.

In this way, when this chip of material 42 is not illuminated, itselectrical resistance remains high, of the order of one hundred megohms.Conversely, when the chip is illuminated by an intense light pulse 44emitted by the source 43, a large number of free charges or carriers arereleased within the chip 42 by the photoconduction effect. The chip thenchanges from an impedance of a few megohms to a few ohms in a very shorttime, typically less than one hundred picoseconds. The then becomesconductive. The time for which the device is conductive (conductiontime), which corresponds to the lifetime of the free carriers,furthermore depends on the thickness of the chip, the nature of thesemiconductor material used and the time for which the semiconductor isilluminated, or more generally the light energy received.

The geometry of the semiconductor chip 42 is in practice selected inorder to avoid reaching the breakdown voltage of the material, knowingthat it is necessary to be able to establish a voltage of a fewkilovolts between the two ends of the chip when it is in the insulatingstate (open switch). This breakdown voltage is a constant of thesemiconductor material, equal for example to 3·10⁵ V/cm for silicon and4·10⁵ V/cm for gallium arsenide. A sufficient thickness is thereforenecessary. Conversely, in order to be able to work at high pulsegeneration frequencies, it is preferable for the chip to change veryrapidly from the insulating state to the conducting state and from theconducting state to the insulating state. This involves using a chip 42of small thickness. Consequently, the choice of the thickness to begiven to the chip 42 results from a compromise which is in particular afunction of the material used, the conduction time of the chip dependingin particular on the nature of the material (recombination time of thephoto-induced carriers).

From the point of view of its operation, the chip 42 itself constitutesa circuit of the RLC type, illustrated by FIG. 5, for which theresistance R_(S) varies from a few megohms when the chip is notilluminated (open state of the switch) to a few ohms when it isilluminated (closed state of the switch). The capacitance C_(S),typically of the order of one picofarad, is a function of the geometryof the chip. The inductance L_(S) is in turn of the order of a few tensof picohenries. This chip therefore represents a resonant circuit, theresonant frequency v_(S) (ν_(S)∝√{square root over(L_(S)·C_(S))}^(−1/2)) of which is typically a few tens of gigahertz.The resistance R_(S) should consequently be adjusted with the aid of theintensity of the light beam applied to the chip in order to suppress anyparasitic oscillation, so as to be able to produce a pulse having asconstant a voltage plate as possible. However, by maintaining a non-zeroresistance when the chip is conductive, the rise time is degraded by afew picoseconds.

According to the invention, the chip of material 42 is thereforedirectly in charge of forming the electrical pulses produced, so thatunlike the generator 31 with which a conventional device is equipped,the principle of which is illustrated by FIG. 3, the generator 41 is aDC voltage generator, or a generator delivering wide pulses which willbe chopped by the switch constituted by the chip 42.

It should be noted that the conduction time of the chip 42, and thedevelopment of the conductivity of the chip during the conduction time,determine the maximum width of the electrical pulse applied to thesample. This duration is in particular a function of the duration of thelight pulse 44 and the nature of the material of the chip. Furthermore,the minimum rise time of the pulse is in turn a function on the one handof the creation rate of free carriers, and on the other hand thestructure of the electronic circuit within which the device is inserted.In this way, by adopting the appropriate structure, a minimal rise timeof the order of a few picoseconds can advantageously be obtained.

Furthermore, the lifetime of the free carriers within the chip 42determines the maximum fall time of the electrical pulse. Consequently,the precise duration of the pulse as well as that of the fall time arecontrolled by virtue of complementary means. The rate of disappearanceof the free carriers may nevertheless be increased, in certainapplications such as the one illustrated by FIG. 12 and described belowin the description, by applying a second light pulse to the chip 42,this second light pulse having a wavelength v′ different to that used toset the chip in conduction. The aim in this case is to obtain anelectrical pulse having a trailing edge with a duration of the order ofthat of the leading edge.

The wavelength of the light pulse 45 is moreover selected, as a functionof the material used, so as to produce the photoconductor effect. Thelight intensity of the source 43 used should also be selected in orderto sufficiently reduce the electrical resistance of the chip when it isin the conducting state (closed switch).

Thus, owing to the size of the pulses generated and the steepness of theleading and trailing edges which it is possible to obtain, the deviceaccording to the invention, particularly in the preferred embodimentmore particularly described in the text above, advantageously replacesthe electrical pulse generator devices conventionally used for atomprobes (cf. FIG. 3).

Furthermore, the use of a chip of semiconductor material as a switchmakes it possible to integrate the switching part of the device producedin this way directly into the vacuum chamber of the probe, so that theswitch can advantageously be placed in the immediate vicinity of theapplication point of the pulse. The adaptation of the switching circuit,and consequently the shape of the pulse produced, can thus be controlledbetter.

The variants of the device according to the invention in the preferredembodiment, for use as an evaporation pulse generator within an atomprobe, are widespread and depend in particular on the arrangement of theelectrode 32 with respect to the sample 33 and the way of producing theoverall electrical connection 46 making it possible to convey theelectrical pulse produced to the electrode 32 and to the sample 33.Various alternative embodiments are presented by way of nonlimitingexamples in the description below. These alternative embodiments takeinto account particularly the fact that the assembly formed by the backelectrode and the sample tip constitutes a circuit of the RLC type,illustrated by FIG. 6, the resistance R_(p) of which varies fromsubstantially zero ohms to one thousand megohms and the capacitanceC_(p) and inductance L_(p) of which are respectively of the order of onehundredth of a picofarad and of the order of one nanohenry. This circuithas a resonant frequency v_(p) (ν_(p)∝√{square root over(L_(p)·C_(p))}^(−1/2)) whose characteristic time is of the order of 1 to5 ps. The device according to the invention should consequently make itpossible to produce pulses whose leading and trailing edges havedurations greater than this value, in order to avoid any resonantoscillation.

FIGS. 7 to 10 illustrate a first variant of the device according to theinvention, as well as the general operating principle of the invention.

FIGS. 7 and 8 schematically present a first embodiment of the deviceaccording to the invention and allow its operation to be detailed. Thisfirst embodiment, as well as the other embodiments presented in thedescription below, are of course presented by way of nonlimitingexamples.

In this alternative embodiment, a voltage generator 71 of impedance R₀is connected to a ring-shaped electrode 73 by means of a transmissionline 72 of length L₀. The electrode is placed facing the end of thesample 76 to be analyzed, which sample is in the form of a tip, itselfplaced at a potential V₀.

The semiconductor chip 75 is placed at the base of the ring 73, which isconnected to a load impedance Z_(c), 74, that closes the line 72.

The propagation line 72 has a characteristic impedance equal to the loadimpedance Z_(c), 74, typically of the order of one hundred ohms, whilethe value of the resistance R₀ of the generator 71 is selected in orderto maximize the reflection of the voltage waves. The value of R₀ isfurthermore less than the impedance R_(off) of the chip 75 in thenonconductive state, typically a few megohms, so as to obtain a maximumvoltage across the terminals of the chip. The relationship between thevarious impedances is therefore established as follows:R₀>>R_(off)>>Z_(c)

The field-effect evaporation of a few atoms of the sample 76 takes placewhen the ring 73, initially at a potential close to the referencepotential, is placed at a negative value with respect to this reference.Control of the evaporation process, on which the resolution of the probeis contingent, requires that this potential is only applied to the ringfor a very brief instant, so that only a few atoms are evaporated.

FIG. 7 presents the device in the absence of stimulation. Thesemiconductor chip 74 is then nonconductive, so that it has a highimpedance R_(off). The ring 73 is then in turn placed at a potentialsubstantially equal to the reference potential, for example the groundpotential (V_(ring)˜0). In this way, the sample 76 being placed only atthe potential V₀, no evaporation takes place.

FIG. 8, on the other hand, presents the device in the presence ofstimulation. This stimulation takes the form of a light pulse whichtriggers the device by making the chip 75 of semiconductor materialconductive so that the latter then has a resistance R_(on) of low orvery low value for a given time.

Consequently, this variation in the value of the resistance R of thechip 75 leads to the appearance at a time t₀ of a voltage step 82 ofvalue V which propagates along the line, with a velocity v_(s), in thedirection of the generator 71 on the one hand and in the direction ofthe load Z_(c), 74, on the other hand. In view of the fact that theimpedance of the chip 75 is substantially zero and the values of theload impedance 74 and the characteristic impedance of the line 72 aresubstantially equal to the same value Z_(c), the amplitude V of thevoltage step is substantially equal to −V_(p)/2. Moreover, the rise timeτ of this step is directly a function of the time taken by the materialto become conductive under the effect of the light pulse. It is of theorder of one picosecond.

When arriving at the input of the generator 71, the voltage step 83 isreflected at R₀ and, in view of the high value of the resistance R₀ withrespect to that of the impedance Z_(c), produces a step 84 of oppositesign with an amplitude substantially equal to V_(p)/2, which in turnpropagates along the line 72 so that, after a time Δt substantiallyequal to 2·L₀/vs, vs representing the propagation velocity of the signalalong the line 74, the incident step 83 and the reflected step 84 areadded together at the ring 73. The voltages applied to the ring 73 thencancel each other in a time (fall time) equivalent to the rise time.Consequently, because of the length L₀ of the line 72 and the fact thatthe ring is located in the vicinity of the chip 75, the ring 73 receivesfirst a voltage step V with a rise time τ then, at the end of the timeΔt₀, an opposite voltage step −V with a fall time also equal to τ. Apulse of amplitude −V_(p)/2 and duration Δt₀ having leading and trailingedges of duration τ is thus applied to the ring 73.

It should be noted that the values of the parameters τ and Δt₀ whichdefine the shape and the duration of the pulse applied to the ring 73can advantageously be controlled. Specifically, the duration τ of theleading and trailing edges is directly a function of the semiconductormaterial employed and the intensity of the light pulse 81 applied. Theduration Δt₀ is in turn determined by the nature and the length of thetransmission line 74.

FIGS. 9 and 10 illustrate by simulation results the advantageous effectsof using the device according to the invention. For this simulation, thesemiconductor chip 75 is modeled by an RLC circuit whose resistanceR_(on) under laser illumination is equal to 10 ohms and whose resistanceR_(off) without illumination is equal to 1 megohm. The inductance L isset here at 5·10⁻¹² henry and the capacitance C at 10⁻¹³ farad.Furthermore, the voltage V_(p) delivered by the generator 71 is set at1000 volts and its resistance R₀ at 33 kilohms. Moreover, the impedanceZ_(c) is set at 100 ohms and the length of the transmission line 72 at 2centimeters. Lastly, the propagation velocity along the line 72 is setat 200 meters per microsecond and the carrier creation time in thesemiconductor constituting the chip 75 is set here at 2 picoseconds.

The oscillogram of FIG. 9 shows that when the semiconductor chip isilluminated by a light pulse, with such a device an electrical pulse 91is produced with a duration substantially equal to 200 picoseconds,having very short leading 92 and trailing 93 edges of equal durations,of the order of a few picoseconds, and an amplitude close to V_(p)/2. Itcan also be seen that the duration of the pulse produced advantageouslydepends only on the propagation time of the voltage step along thepropagation line 72.

The illustration of FIG. 10 presents on a single diagram, for ahomogeneous sample consisting of elements with mass equal to 30 AMU, themass spectra 101 and 102 obtained respectively by using a pulsegenerator of known type from the prior art and a pulse generatorproduced by means of the device according to the invention, asillustrated by FIGS. 7 and 8. As illustrated in the figure, using thedevice according to the invention advantageously makes it possible toproduce pulses whose characteristics improve the overall performance ofthe atom probe in which it is integrated. This improvement in terms ofmass resolution is demonstrated on the figure by attenuation of the baseof the spectrum by at least one order of magnitude with respect to thespectrum of the same sample obtained by using known conventional means.

FIG. 11 schematically presents a second alternative embodiment of thedevice according to the invention.

In this variant, the evaporation pulse generator is arranged so that thesample to be analyzed is directly integrated into the assembly insteadof the ring electrode 73. The potential reference of the device isplaced at the potential V₀ applied only to the sample in the previousembodiment, the assembly consisting of the device and the sample thusbeing placed at a common potential equal to V₀.

The voltage generator is furthermore configured in order to deliver avoltage +V_(p) which can be added to the voltage V₀ applied to thesample.

Thus, at rest i.e. in the absence of illumination, the chip ofsemiconductor material 74 is then nonconductive, so that it has a highimpedance R_(off). The sample 76 is then placed at a potentialsubstantially equal to the reference potential V₀, so that no meltingtakes place.

On the other hand, during operation i.e. when the chip of semiconductormaterial 74 is subjected to illumination and therefore has a very lowimpedance R_(on), the sample 76 is subjected to a voltage pulse whichplaces it throughout its duration at a potential substantially equal toV₀+V_(p), which causes the desired evaporation.

In this second alternative embodiment, as in the previous one, theduration of the pulse is determined by the length of the line 72 whichmakes it possible to apply the voltage delivered by the generator 71 tothe sample 76. By using this second arrangement in an atom probe, thesame advantages as those obtained by means of the previous arrangementare therefore obtained in terms of resolution.

FIG. 12 schematically presents a third alternative embodiment of thedevice according to the invention.

The variant described here differs from the previous ones in that theduration of the electrical pulse produced, as well as the durations ofits leading and trailing edges, are obtained simply by acting on thechip of semiconductor material 75.

The chip 75 is rendered conductive (R=R_(on)) by applying a first lightpulse 121, then is made nonconductive again (R=R_(off)) at the end of atime Δt₀ by applying a second light pulse 122 with a differentwavelength to the first light pulse. An electrical pulse of duration Δt₀is thus produced.

The action of the first light pulse 121 consists, as before, in creatingphoto-induced free carriers in the semiconductor material. The secondlight pulse 122, for its part, acts on the semiconductor by activatingthe recombination sites of the semiconductor so as to causequasi-instantaneous destruction of the free carriers. According to thisembodiment, the semiconductor material constituting the chip is selectedso as to react to the two wavelengths in question.

According to the invention, the time interval Δt₀ between the two lightpulses 121 and 122 is produced by any known means, for example anoptical delay line.

An electrical pulse is thus obtained whose duration is a function of thetime interval between the two light pulses and whose leading andtrailing edges have durations, of the order of a few picoseconds, whichdepend only on the time taken to make the semiconductor conductive andthe recombination time of the free carriers created by the first lightpulse.

1. A tomographic atom probe having means for applying an electricalevaporation pulse of amplitude V_(p) and duration Δt₀ to an end of asample which is placed at a potential V₀, said tomographic atom probecomprising: an electrode which is placed at an initial potential V_(i)and is configured and arranged to apply the electrical pulse to thesample; a voltage generator to produce a voltage necessary for producinga pulse of amplitude V_(p), the voltage generator being connected to theelectrode by means of an electrical connection which can be open orclosed; means for causing closure of the electrical connection in agiven time τ so as to apply a voltage step V_(p) to the electrode, thesemeans comprising a chip of semiconductor material placed on theelectrical connection between the generator and the electrode, in thevicinity of the electrode, and a first source emitting light pulses ofwavelength λ₁ to the semiconductor chip, said chip becoming conductiveand closing the electrical connection when it is illuminated with alight pulse of wavelength λ₁, the conduction time being a function ofthe duration of the light pulse applied; means for applying a voltagestep of amplitude (−V_(p)) to the electrode at the end of a time Δt₀after closure of the electrical connection, so as to place the electrodeat the potential V_(i), said means being configured so that the voltagestep is applied in a time τ′ substantially equal to τ.
 2. Thetomographic atom probe according to as claimed in claim 1, wherein themeans for applying a voltage step of amplitude −V_(p) to the electrodeat the end of a time Δt₀ after closure of the circuit consist of atransmission line with a characteristic impedance Z_(c) connecting thegenerator to the chip, terminated downstream of the electrode by animpedance equal to Z_(c).
 3. The tomographic atom probe according toclaim 2, wherein the length L₀ of the transmission line is determined bythe value of the time interval Δt₀ in question, said time interval beingequal to the duration of the electrical pulse produced.
 4. Thetomographic atom probe as claimed in according to claim 1, wherein themeans for applying a voltage step of amplitude −V_(p) to the electrodeat the end of a time Δt₀ after closure of the circuit consist of asecond source emitting light pulses of wavelength λ₂ onto thesemiconductor chip, said chip becoming insulating and opening theelectrical connection when it is illuminated with a light pulse ofwavelength λ₂.
 5. The tomographic atom probe according to claim 4,wherein the emission of a light pulse of wavelength λ₁ and the emissionof a light pulse of wavelength λ₂ are separated from one another by atime interval Δt₀, which determines the duration of the electrical pulseproduced.
 6. The tomographic atom probe according to claim 5, whereinthe electrode on which the electrical pulse is produced consists of thesample itself.
 7. The tomographic atom probe according to claim 5,wherein the electrode on which the electrical pulse is produced ispositioned facing an end of the sample to which the evaporation pulsesare applied.
 8. The tomographic atom probe according to claim 4, whereinthe electrode on which the electrical pulse is produced consists of thesample itself.
 9. The tomographic atom probe according to claim 4,wherein the electrode on which the electrical pulse is produced ispositioned facing an end of the sample to which the evaporation pulsesare applied.
 10. The tomographic atom probe according to claim 1,wherein the electrode on which the electrical pulse is produced ispositioned facing an end of the sample on which the evaporation pulsesare applied.
 11. The tomographic atom probe according to claim 1,wherein the electrode on which the electrical pulse is produced consistsof the sample itself.